A. Field of Invention
The present invention relates to MEMS devices, and more particularly, to cryogenic inertial MEMS devices.
B. Description of Related Art
Micro mechanics, micro-machines, or more commonly, Micro-Electro-Mechanical Systems (MEMS) are an integration of mechanical elements, such as sensors and actuators, and/or electronics on a common substrate through the utilization of micro-fabrication technology. While the electronics are fabricated using Integrated Circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), micro-mechanical components are fabricated using compatible xe2x80x9cmicro-machiningxe2x80x9d processes that selectively etch away parts of a silicon wafer or add new structural layers to form mechanical and electromechanical devices.
Micro mechanical sensors include a mechanical structure and a sensing device. The mechanical structure is typically deformed and the sensing device transforms the deformation into an electrical signal. The mechanical structure deformation depends upon the shape of the structure, and also on the mechanical properties of the structure, such as Young""s Modulus, the Poisson ratio, and mechanical load distribution. Furthermore, environmental parameters, such as pressure, temperature, humidity, acceleration, rotation, etc., dictate a deformation of the mechanical structure. Mechanical properties and environmental parameters such as these can largely affect operation of a MEMS device due to a small size and shape of the MEMS device.
The sensing device within micro mechanical sensors may include electrically conducting bodies in close proximity with each other. Altering an arrangement of the electrically conducting bodies causes a change in capacitance that can be detected by a variation in voltage. Alternatively, MEMS devices may also operate based on temperature and heat conduction effects. Two metal plates may be placed in close proximity to each other and a heat transfer through a medium in a gap between the plates can be measured by a temperature resistance effect observed on the plates. Thus, methods of operation can be affected due to inherent electrical and mechanical properties of a MEMS.
MEMS operation presents obstacles due to these challenges present within the micro-machine domain. Empirical laws known to be true but insignificant at a macro level can be significant at a micro level. For example, a typical electrical conductor, such as gold, includes intrinsic electronic noise, which leads to inherent resistance, which can dominate operation of MEMS.
MEMS devices also include intrinsic fluctuations of operation due to thermal dynamical properties of the MEMS. For example, in operation of vibratory MEMS, such as a MEMS tuning fork gyroscope, the performance of the MEMS may be limited due to physical limitations of materials used within the MEMS, such as electrical conductors. For instance, Johnson noise (e.g., thermal noise) may be present within electrical conductors. Johnson noise is an inherent property among electrical conductors because it is substantially always present due to a random thermal motion of electrons. The Johnson noise power per unit bandwidth is proportional to absolute temperature and is independent of frequency, therefore operation of a MEMS at higher temperatures induces a larger Johnson noise. In addition, typical operation of a MEMS may be affected due to the Seebeck effect. The Seebeck effect produces a voltage between dissimilar conducting materials in the presence of a non-uniform temperature. In this situation, fluctuations in the temperature can produce voltage noise in MEMS devices. Finally, Brownian motion is present in substantially all devices due to fluctuating thermal mechanical forces. In devices with a small mass, such as MEMS devices, Brownian motion is especially significant. Brownian motion is a fundamental limitation to the ability to measure the position of a MEMS device. For example, the output of a MEMS gyroscope or MEMS accelerometer is determined by measuring the distance between two capacitor plates, at least one of which is located on the MEMS device.
Consequently, typical operation of a MEMS may be limited by inherent mechanical and electrical properties. As such, existing MEMS may not be able to meet manufacturing requirements and/or desired applications of MEMS. Thus, it would be desirable to provide a MEMS device that is able to overcome inherent mechanical and electrical property limitations.
In view of the above, some of the problems within MEMS operation associated with thermal and mechanical noise are overcome. In an exemplary embodiment, a method of operating an inertial Micro-Electro-Mechanical System (MEMS) device at a cryogenic temperature is provided. The inertial MEMS device may include cryogenic and/or super conducting materials to enhance operation at cryogenic temperatures. In addition, a method of operating an inertial Micro-Electro-Mechanical-System (MEMS) device is provided by providing a housing and an inertial MEMS device coupled to a pre-amplifier. The method further includes co-locating the inertial MEMS device and the pre-amplifier within a close proximity of the housing and operating the inertial MEMS device at cryogenic temperatures.
These as well as other features and advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.